Steven M. Pike

Fukushima-derived radionuclides in the ocean and biota off Japan

The Tōhoku earthquake and tsunami of March 11, 2011, resulted in unprecedented radioactivity releases from the Fukushima Dai-ichi nuclear power plants to the Northwest Pacific Ocean. Results are presented here from an international study of radionuclide contaminants in surface and subsurface waters, as well as in zooplankton and fish, off Japan in June 2011. A major finding is detection of Fukushima-derived 134Cs and 137Cs throughout waters 30–600 km offshore, with the highest activities associated with near-shore eddies and the Kuroshio Current acting as a southern boundary for transport. Fukushima-derived Cs isotopes were also detected in zooplankton and mesopelagic fish, and unique to this study we also find 110mAg in zooplankton. Vertical profiles are used to calculate a total inventory of ∼2 PBq 137Cs in an ocean area of 150,000 km2. Our results can only be understood in the context of our drifter data and an oceanographic model that shows rapid advection of contaminants further out in the Pacific. Importantly, our data are consistent with higher estimates of the magnitude of Fukushima fallout and direct releases [Stohl et al. (2011) Atmos Chem Phys Discuss 11:28319–28394; Bailly du Bois et al. (2011) J Environ Radioact, 10.1016/j.jenvrad.2011.11.015]. We address risks to public health and marine biota by showing that though Cs isotopes are elevated 10–1,000× over prior levels in waters off Japan, radiation risks due to these radionuclides are below those generally considered harmful to marine animals and human consumers, and even below those from naturally occurring radionuclides.

Differences in seawater particulate organic carbon concentration in samples collected using small- and large-volume methods : the importance of DOC adsorption to the filter blank

Particulate organic carbon (POC) data collected by small-volume (∼1–2 l) bottle filtration and large-volume (∼100–600 l) methods are compared for samples from the central Arctic, Equatorial Pacific, Equatorial and South Atlantic, Gulf of Maine, and Narragansett Bay. Small-volume samples were collected using Niskin® and Go-Flo® bottles and large-volume samples were collected using in situ pumps and large-volume bottle filtration. Results indicate that small-volume bottle POC data are often greater than large-volume results, by as much as 2–4 times, in regions with low POC concentration (<∼5 μM). The implication is that POC concentrations determined by small-volume bottle filtration in regions characterized by low POC concentrations, i.e., in the majority of surface open ocean and deep waters, may be erroneously high. We suggest the most likely explanation is adsorption of DOC to the filter, which is rarely quantified yet can significantly increase the filter blank. The magnitude of DOC adsorption was assessed using coastal seawater by determining the y-intercept of a plot of the organic carbon retained by a glass-fiber filter against the volume filtered. The intercept was approximately two-fold greater than the precombusted filter blank, which we attribute to DOC adsorbed to the filter. Thus, when seawater POC concentrations are similar to, or less than, the precombusted filter blank, not correcting for the true in situ blank can result in erroneously high POC concentrations. To avoid this artifact, we recommend using large-volume sampling methods, which result in a greater quantity of POC per unit area of the filter relative to the filter blank. When large-volume filtration is not possible, we recommend a simple method to evaluate the true in situ filter blank.

Investigating the carbon cycle in the Gulf of Maine using the natural tracer thorium 234

The naturally occurring radionuclide 234Th (t1/2 = 24.1 days) was used to examine the organic carbon cycle in the Gulf of Maine. A seasonal study (March, June, and September 1995) was conducted in the central Gulf of Maine in Wilkinson and Jordan Basins, and a regional survey, which included the Scotian Shelf, was conducted during August-September 1997. Particulate organic carbon (POC) export (particulate export production) was estimated from a three-dimensional steady state model of 234Th flux combined with measurements of the POC/234Th ratio on >53-μm particles. The POC export for this region was seasonally variable; average values ranged from 15 to 34 mmol C m−2 d−1, between 11% and 25% of the regionally integrated primary production. The Gulf of Maine was a net source (to the Mid-Atlantic Bight) of dissolved organic carbon (2.4 mmol C m−2 d−1) amounting to ∼2% of carbon uptake rates. Organic carbon burial in the sediments was a minor fraction of the primary production, averaging 1.6 mmol C m−2 d−1. Though only a fraction of total export production was buried in the sediments, these estimates close the budget for organic carbon in the Gulf of Maine. An implication is that off-shelf export may not be as important as previously estimated in this shelf region.

Size-fractionated plutonium isotopes in a coastal environment

We have examined the distribution of individual Pu isotopes (239Pu, 240Pu, and 241Pu) in seawater from the Gulf of Maine (GOM). Samples were size-fractionated with a 1 kD cross-flow ultrafiltration (CFF) membrane. Subfractioned samples were radiochemically purified and Pu isotopes were analyzed using a three-stage thermal ionization mass spectrometer (TIMS). To our knowledge, this is the first time that both size class and Pu isotopic data have been obtained for seawater samples. Within measurement uncertainties a single 240Pu/239Pu atom ratio of 0.18 was found for all sample collection depths and sample size fractions. This signifies a current, single Pu source in GOM waters, namely global fallout, and suggests that no measurable isotopic fractionation occurred during CFF processing. The majority of Pu was found in the low molecular weight fraction (< 1 kD). Colloidal Pu varied from 8% of the total in surface waters to < 1% in the deepest (250 m) seawater sample. Evidence suggests that the vertical distribution of Pu in GOM is primarily controlled by conservative mixing processes. The high Pu fraction found in the low molecular size fraction implies that most of the Pu is in the non-particle-reactive oxidized fraction, and is consistent with the conservative Pu behavior. The activity levels are in agreement with other studies which show a slow decrease in Pu with time due to continued mixing and relatively slow particle removal.

Use of Poretics® 0.7 μm pore size glass fiber filters for determination of particulate organic carbon and nitrogen in seawater and freshwater

Abstract Poretics® glass fiber filters of 0.7 μm nominal pore size (GF-75) were evaluated for determination of particulate organic carbon and nitrogen in seawater and freshwater. Carbon and nitrogen blanks determined by CHN analysis of precombusted 25 mm filters were, respectively, 0.8 ± 0.3 and 0.02 ± 0.01 μmol for Poretics® GF-75 filters, compared to 1.7 ± 0.7 and 0.19 ± 0.12 μmol for Whatman® GF/F filters. These results suggest that GF-75 filters are characterized by lower blanks and better analytical precision for determination of carbon and nitrogen than GF/F filters. Comparable POC and PON concentrations were obtained using GF-75 and GF/F filters on paired analyses of samples from the Gulf of Maine, Narragansett Bay, and a freshwater creek. Depth profiles of POC and PON determined using GF-75 filters are presented for the Gulf of Maine and the Canada Basin, Arctic Ocean. Results reported in this study demonstrate that the Poretics® GF-75 filter is a suitable, cost effective, alternative to the widely used Whatman® GF/F filter.

Reassessment of 90Sr, 137Cs, and 134Cs in the Coast off Japan Derived from the Fukushima Dai-ichi Nuclear Accident

The years following the Fukushima Dai-ichi nuclear power plant (FDNPP) accident, the distribution of (90)Sr in seawater in the coast off Japan has received limited attention. However, (90)Sr is a major contaminant in waters accumulated within the nuclear facility and in the storage tanks. Seawater samples collected off the FDNPP in September 2013 showed radioactive levels significantly higher than pre-Fukushima levels within 6 km off the FDNPP. These samples, with up to 8.9 ± 0.4 Bq·m(-3) for (90)Sr, 124 ± 3 Bq·m(-3) for (137)Cs, and 54 ± 1 Bq·m(-3) for (134)Cs, appear to be influenced by ongoing releases from the FDNPP, with a characteristic (137)Cs/(90)Sr activity ratio of 3.5 ± 0.2. Beach surface water and groundwater collected in Sendai Bay had (137)Cs concentrations of up to 43 ± 1 Bq·m(-3), while (90)Sr was close to pre-Fukushima levels (1-2 Bq·m(-3)). These samples appear to be influenced by freshwater inputs carrying a (137)Cs/(90)Sr activity ratio closer to that of the FDNPP fallout deposited on land in the spring of 2011. Ongoing inputs of (90)Sr from FDNPP releases would be on the order of 2.3-8.5 GBq·d(-1) in September 2013, likely exceeding river inputs by 2-3 orders of magnitude. These results strongly suggest that a continuous surveillance of artificial radionuclides in the Pacific Ocean is still required.

Tracking the Fate of Particle Associated Fukushima Daiichi Cesium in the Ocean off Japan.

A three year time-series of particle fluxes is presented from sediment traps deployed at 500 and 1000 m at a site 115 km southeast of Fukushima Daiichi Nuclear Power Plant (FDNPP). Results show a high fraction of lithogenic material and mass flux peaks that do not align between the trap depths, suggesting a lateral source of sediments. Fukushima cesium-137 and cesium-134 were enhanced in flux peaks that, given variations in trap (137)Cs/(210)Pbex ratios, are characteristic of material derived from shelf and slope sediments found from <120 to >500 m. These lateral flux peaks are possibly triggered by passing typhoons. The Cs fluxes are an order of magnitude higher than were previously reported for the trap located 100 km due east of FDNPP. We attribute this large difference to the position of our trap under the southeasterly currents that carry contaminated waters and resuspended sediments away from FDNPP and into the Pacific. These higher Cs sedimentary fluxes offshore are still small relative to the inventory of Cs currently buried nearshore. Consequently, we do not expect them to effect any rapid decrease in Cs levels for the coastal sediments near FDNPP that have been linked to enhanced Cs in demersal fish.

Lingering radioactivity at the Bikini and Enewetak Atolls

We made an assessment of the levels of radionuclides in the ocean waters, seafloor and groundwater at Bikini and Enewetak Atolls where the US conducted nuclear weapons tests in the 1940s and 50s. This included the first estimates of submarine groundwater discharge (SGD) derived from radium isotopes that can be used here to calculate radionuclide fluxes in to the lagoon waters. While there is significant variability between sites and sample types, levels of plutonium (239,240Pu) remain several orders of magnitude higher in lagoon seawater and sediments than what is found in rest of the worlds oceans. In contrast, levels of cesium-137 (137Cs) while relatively elevated in brackish groundwater are only slightly higher in the lagoon water relative to North Pacific surface waters. Of special interest was the Runit dome, a nuclear waste repository created in the 1970s within the Enewetak Atoll. Low seawater ratios of 240Pu/239Pu suggest that this area is the source of about half of the Pu in the Enewetak lagoon water column, yet radium isotopes suggest that SGD from below the dome is not a significant Pu source. SGD fluxes of Pu and Cs at Bikini were also relatively low. Thus radioactivity associated with seafloor sediments remains the largest source and long term repository for radioactive contamination. Overall, Bikini and Enewetak Atolls are an ongoing source of Pu and Cs to the North Pacific, but at annual rates that are orders of magnitude smaller than delivered via close-in fallout to the same area.